This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to 0001522-2 filed in Sweden on Apr. 27, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to a method and an apparatus for compacting a powder material into a homogenous article.
In processes for manufacturing articles, such as bearing rings, bushes, solid bodies, etc, by compacting powder materials in a molding cavity it is essential that the powder be evenly distributed within the molding cavity before the start of the pressing operation. However, it is difficult to fill a molding cavity with powder so that the particles in the powder are uniformly distributed and so that the upper surface of the powder material filling the molding cavity is horizontal. Moreover, if the pressing operation takes place with a very high pressing rate, so that the pressing time is in the order of a few milliseconds, there is a great risk that air will become entrapped in the compacted article, disturbing the homogeneity thereof.
An object of the present invention is to solve these problems by providing a method and an apparatus for compacting a powder material into a homogenous article in which the particles in the powder are evenly distributed within the molding cavity and the upper surface of the powder in the molding cavity is horizontal before the start of the pressing step.
This object, and others are accomplished by a method for compacting powder material to form a homogenous article, comprising the steps of; placing the powder material in a molding cavity connected to a gas source, blowing gas into the lower end of the molding cavity so that the particles in the powder material are suspended in a gas stream; sealing the upper end of the molding cavity by an upper pressing punch, connecting the lower end of the molding cavity to a vacuum source; sealing the connection to the vacuum source by moving a lower punch relative to the lower end portion of the molding cavity; and thereafter compacting the powder material with the assistance of the pressing punch.
In a preferred embodiment the vacuum source creates a sub-pressure in the molding cavity of at least half the atmospheric pressure.
The invention relates also to an apparatus for compacting a powder material to form a homogenous article comprising a molding cavity having vertical side walls and open ends, and an upper and a lower punch having the same cross-sectional area as the molding cavity and being movable into and out of the molding cavity, an opening arranged in the side wall of the molding cavity in the lower end part thereof, said opening being connected to a device for blowing gas into and a device for sucking gas out of the molding cavity, the lower punch is movable relative to the molding cavity from a first position in which an upper end surface of the lower punch is located below the opening to a second position in which the opening is covered by a side wall of the lower punch.
In a preferred embodiment a gas-pervious membrane covers the opening, said membrane being impervious to the smallest of the particles in the powder material to be placed in the molding cavity. Preferably, the vertical walls of the molding cavity comprise an inner wall of a matrix having the form of a hollow cylinder, and the side wall of a cylindrical core pin concentrically located in the inner space defined by the hollow matrix. The upper and lower punch preferably have lower end surface and an upper end surface, respectively, comprising an annular flange fitting into the annular molding cavity. The opening preferably connected to a device for blowing gas therein and a device for sucking gas out of the molding cavity comprising an annular groove in the lower end portion of the inner wall of the matrix and/or the side wall of the core pin.